Group key management approach based on linear geometry

ABSTRACT

A group key management approach based on linear geometry is disclosed. The approach includes the following steps: step 1: a group controller selects a mapping f and a finite field F; each group member selects a m-dimensional private vector over the finite field F, and sends it to the group controller via secure channel; step 2: the group controller selects a mapping parameter in the finite field F randomly, and maps the private vectors of all the group members into a new set of vectors by using the mapping f according to the mapping parameter; step 3: the group controller selects a random number k in the finite field F as a group key, and constructs a system of linear equations by using the new set of vectors and the group key; the group controller computes the central vector, and sends the central vector and the mapping parameter to all the group members via open channel; step 4: after the group members receive the central vector and the mapping parameter, the private vector of each group member is mapped to a new vector in a vector space according to the mapping parameter, and the group key is obtained by calculating the inner product of the new vector and the central vector. This invention requires small memory and little computation, has high security property, and is effective against brute-force attacks.

FIELD OF THE INVENTION

The present invention relates to the field of group key management in network security technology, and in particular to a group key management approach based on linear geometry.

BACKGROUND OF THE INVENTION

With the rapid development of Internet technology and the popularization of multicast, group-oriented applications, such as video conference, network games, and video on demand, etc., play more and more important roles. How to protect the communication security of these applications is a critical problem. A secure group communication system should not only provide data confidentiality, user authentication, and information integrity, but also good scalability. For a secure group communication system, a secure, efficient, and robust group key management approach is essential.

These days, there are various approaches in key management of the secure group communication. The typical schemes are for example Group Key Management Protocol (GKMP), Secure Lock (SL), Logical Key Hierarchy (LKH), etc.

Group Key Management Protocol (GKMP) is a scheme that extends directly from unicast to multicast communication. In this scheme, it is assumed that a secure channel exists between the Group Controller (GC) and each group member. Initially, the GC selects a group key K₀ and distributes this key to all group members via the secure channel. When a new member joins, the GC selects a new group key K_(N), and encrypts the new group key with the old group key to obtain K′=E_(K) _(N) (K₀), then broadcasts K′ to the entire group. Moreover, the GC sends K_(N) to the joining new member via the secure channel between the GC and the new member. Obviously, this scheme is not scalable, and there is no solution to keep the forward secrecy property when a member leaves the group, except to recreate an entirely new group without containing that member.

The Secure Lock (SL) scheme takes advantage of Chinese Remainder Theorem (CRT) to construct a secure lock to combine all the re-keying messages into one when the group key is updated. However, the CRT is a time-consuming operation. The SL scheme is efficient only when the number of users in a group is small, since the time to compute the lock and the length of the lock (hence the transmission time) is proportional to the number of users.

The Logical Key Hierarchy (LKH) scheme adopts tree structure to organize keys. The GC maintains a virtual tree, and the nodes in the tree are assigned keys. The key held by the root of the tree is the group key. The internal nodes of the tree hold key encryption keys (KEK). Keys at leaf nodes are possessed by different members. Each member is assigned the keys along the path from its leaf to the root. When a member joins or leaves the group, its parent node's KEK and all KEKs held by nodes in the path to the root should be changed. Therefore, the number of keys which need to be changed for a joining or leaving is O(2×log₂ n), and the number of encryptions is O(2×log₂ n). If a great deal of members join or leave the group, then the re-keying overhead will increase proportionally to the number of members changed. In addition, there are some other schemes that adopt tree structures, for example, OFT (One-way Function Tree), OFCT(One-way Function Chain Tree), Hierarchical a-ary Tree with Clustering, Efficent Large-Group Key, etc. These schemes are similar to the LKH or can be regarded as improvements to the LKH.

SUMMARY OF THE INVENTION

It is a first object of the present invention to provide a group key management approach based on linear geometry, which requires small memory and little computation, and is effective against brute-force attacks.

It is a second object of the present invention to provide a further group key management approach based on linear geometry, which requires small memory and little computation, has high security property, and is effective against brute-force attacks.

The first object of the present invention is achieved by the following technical solution:

a group key management approach based on linear geometry comprises the following steps:

step 1: a group controller selects a mapping f and a finite field F for use by a group (all computations in the group are performed over the finite field); suppose there are n members in the group, each group member selects a m-dimensional private vector over the finite field F, and sends the m-dimensional private vector to the group controller via secure channel; the group controller assigns and sends a serial number to each member, and receives the m-dimensional private vector of each member and keeps secret; wherein m, n are positive integers, and 2≦m≦n;

step 2: the group controller selects a mapping parameter in the finite field F randomly, and maps private vectors of all the members into a new set of vectors via the mapping f according to the mapping parameter, if the set of vectors is linearly dependent, then the mapping parameter is reselected to perform remapping, or return to the step 1 to make each member reselect a private vector, until the new set of vectors is linearly independent;

step 3: the group controller selects a random number in the finite field F as a group key, and constructs a system of linear equations by using the new set of vectors and the group key; the group controller computes the unique solution of the system of linear equations which is called central vector, and the central vector and the mapping parameter are broadcasted or multicasted by the group controller to all the group members via open channel;

step 4: after the group members receive the central vector and the mapping parameter, the private vector of each group member is mapped to a new vector in the vector space according to the mapping parameter, and the group key is obtained by calculating the inner product of the new vector and the central vector.

Preferably, the step 1 is implemented by the following:

the group controller selects a mapping f and a finite field F for use by a group; each member selects a m-dimensional private vector over the finite field F, and sends the m-dimensional private vector to the group controller via secure channel; wherein m is a positive integer, and 2≦m≦n;

the group controller assigns and sends a serial number u_(i) to each member, and receives the m-dimensional private vector V_(i)=(v_(i,1),v_(i,2), . . . , v_(i,m)) of each member and keeps secret, wherein i=1, . . . , n;

the step 2 is implemented by the following:

the group controller selects a mapping parameter r in the finite field F randomly, and maps private vectors V_(i)=(v_(i,1),v_(i,2), . . . , v_(i,m)) of all the members u_(i) into a new set of vectors via the mapping f according to the mapping parameter r:

for each member u_(i), where i=1,2, . . . , m:

x_(i, 1) = f(v_(i, 1), r) x_(i, 2) = f(v_(i, 2), r) … x_(i, m) = f(v_(i, m), r)

for each member u_(i), where i=m+1, . . . , n:

x_(i, 1) = f(v_(i, 1), r) x_(i, i − m + 2) = f(v_(i, 2), r) … x_(i, i) = f(v_(i, m), r)

in this way, the group controller obtains a new set of vectors over the finite field F:

for each member u_(i), where i=1,2, . . . , m:

X _(i)=(x _(i,1) ,x _(i,2) , . . . ,x _(i,n)), wherein x _(i,m+1) , . . . ,x _(i,n) are all 0;

for each member u_(i), where i=m+1, . . . , n:

X _(i)=(x _(i,1) ,x _(i,2) , . . . ,x _(i,n)), wherein x _(i,2) , . . . ,x _(i,i−m+1) and x _(i,i+1) , . . . ,x _(i,n) are all 0;

the group controller judges whether X₁, X₂ . . . , X_(n) are linearly independent, and if they are linearly independent, then proceed to the step 3; otherwise, return to the step 2, or return to the step 1 to allow the group member to reselect a private vector; (since r is a random number, it is easily to obtain a set of vectors X₁, X₂ . . . , X_(n) that are linearly independent);

the step 3 is implemented by the following:

the group controller selects a random number K in the finite field F as a group key, and constructs a system of linear equations by using the new set of vectors and the group key:

suppose a₁,a₂, . . . , a_(n) are unknown parameters, and the group controller solves the central vector A=(a₁,a₂, . . . , a_(n)) from the following system of linear equations:

$\left\{ {\begin{matrix} {{{x_{1,1}a_{1}} + {x_{1,2}a_{2}} + \ldots + {x_{1,n}a_{n}}} = k} \\ {{{x_{2,1}a_{1}} + {x_{2,2}a_{2}} + \ldots + {x_{2,n}a_{n}}} = k} \\ \ldots \\ {{{x_{n,1}a_{1}} + {x_{n,2}a_{2}} + \ldots + {x_{n,n}a_{n}}} = k} \end{matrix}\quad} \right.$

this system of linear equations can be represented in vector form: X×A^(T)=K^(T)

wherein T represents the transpose of the matrix, the vector K=(k,k, . . . k), the vector A=(a₁,a₂, . . . , a_(n)), and the matrix

${X = {\begin{bmatrix} X_{1} \\ X_{2} \\ \ldots \\ X_{n} \end{bmatrix} = \begin{bmatrix} x_{1,1} & x_{1,2} & \ldots & x_{1,n} \\ x_{2,1} & x_{2,2} & \ldots & x_{2,n} \\ \ldots & \ldots & \ldots & \ldots \\ x_{n,1} & x_{n,2} & \ldots & x_{n,n} \end{bmatrix}}};$

since X₁,X₂ . . . , X_(n) are linearly independent vectors and the determinant of the coefficient matrix |X|≠0, this system of linear equations has the unique solution.

the central vector A=(a₁,a₂, . . . , a_(n)) and the mapping parameter r are broadcasted or multicasted by the group controller to all the group members via open channel;

the step 4 is implemented by the following:

after the group members receive the central vector A=(a₁,a₂, . . . , a_(n)) and the mapping parameter r, the private vector of each group member u_(i) is mapped to a new vector in the vector space according to the mapping parameter r:

for the group member u_(i), where i=1,2, . . . , m:

x_(i, 1) = f(v_(i, 1), r) x_(i, 2) = f(v_(i, 2), r) … x_(i, m) = f(v_(i, m), r)

that is to say:

X_(i)=(x_(i,1),x_(i,2), . . . ,x_(i,n)), wherein x_(i,m+1), . . . , x_(i,n) are all 0;

for the member u_(i), where i=m+1, . . . , n:

x_(i, 1) = f(v_(i, 1), r) x_(i, i − m + 2) = f(v_(i, 2), r) … x_(i, i) = f(v_(i, m), r)

that is to say:

X_(i)=(x_(i,1),x_(i,2), . . . , x_(i,n)), wherein x_(i,2), . . . , x_(i,i−m+1) and x_(i,i+1), . . . , x_(i,n) are all 0;

then the group member u_(i) can calculate the group key k via the equation below, i.e. the inner product of the vector X_(i) and the central vector A=(a₁,a₂, . . . , a_(n))

k=X _(i) ×A ^(T) =x _(i,1) a ₁ +x _(i,2) a ₂ + . . . +x _(i,n) a _(n), wherein T is the transpose of the matrix;

when a new member joins in the group, the group key management approach based on linear geometry further includes:

step 5, when new members join in the group, each new member selects a m-dimensional private vector over the finite field F, and sends the m-dimensional private vector to the group controller via secure channel; the group controller assigns and sends a serial number to each new member, and receives the m-dimensional private vector of each new member and keeps secret;

repeat the steps 2 to 4.

When group members need to leave the group, the group key management approach based on linear geometry further includes:

step 5, when group members need to leave the group, each member that needs to leave the group applies to the group controller for leaving the group; the group controller deletes the private vectors of the leaving members, and reassigns serial numbers for the remaining members according to the size order of the subscripts of the current members, and sends the serial numbers to all members via open channel;

repeat the steps 2 to 4.

When there are new members want to join in the group and members want to leave the group simultaneously, the group key management approach based on linear geometry further includes:

step 5, when there are new members want to join the group and members want to leave simultaneously, each new member selects a m-dimensional vector over the finite field F and sends the m-dimensional vector to the group controller via secure channel; the members want to leave the group apply to the group controller for leaving the group, and then the group controller deletes the private vector of each leaving member and reassigns serial numbers to the remaining members according to the size order of the subscripts of the current members, and assigns a serial number to each new member; the m-dimensional private vector of the new member is received by the group controller and kept secret; then the subscripts of all members are broadcasted to all members via open channel;

repeat the steps 2 to 4.

Preferably, the m-dimensional private vector is a two-dimensional private vector.

Preferably, the group key management approach based on linear geometry further includes auto update: if no member joins or leaves the group in a preset period, then the group controller will update the group key periodically, the group controller reselects the mapping parameter and the group key and calculates the central vector, and the central vector and the mapping parameter are broadcasted or multicasted to all members by the group controller via open channel.

Preferably, suppose the mapping f can be represented by z=f(w,y), where w,y,zεF, the main function of the mapping f is randomization, and the mapping f conforms to the following characteristics:

1) it is easy to calculate z=f (w,y) if w, y are known;

2) it is difficult to calculate w if only z and y are known; if z and w are known, it is also difficult to compute y from z=f(w,y); it is difficult to compute w_(i) from z_(i)=f (w_(i),y_(i)), though a series of z_(i) and y_(i) is obtained; and it is also difficult to compute y_(i) from z_(i)=f (w_(i),y_(i)), though a series of z_(i) and w_(i) is obtained.

The second object of the present invention is achieved by the following technical solution:

a group key management approach based on linear geometry comprises the following steps:

step 1: a group controller selects a mapping f and a finite field F for use by a group (all computations in the group are performed over the finite field); suppose there are n members in the group, each member selects a m-dimensional private vector over the finite field F, and sends the m-dimensional private vector to the group controller via secure channel; the group controller assigns and sends a serial number to each member, and receives the m-dimensional private vector of each member and keeps secret; wherein m, n are positive integers, and 2≦m≦n+1;

step 2: after the group controller receives the private vectors of all members, the group controller itself also selects a m-dimensional private vector, and the group controller selects a mapping parameter in the finite field F randomly, and maps the private vector of the group controller and the private vectors of all the members into a new set of vectors via the mapping f according to the mapping parameter; if the set of vectors is linearly dependent, then the mapping parameter is reselected to perform remapping, or return to step 1 to make each member reselect a private vector, until the new set of vectors is linearly independent;

step 3: the group controller selects a random number in the finite field F as a group key, and constructs a system of linear equations by using the new set of vectors and the group key; the group controller computes the unique solution of the system of linear equations which is called central vector, and the central vector and the mapping parameter are broadcasted or multicasted by the group controller to all the group members via open channel;

step 4: after the group members receive the central vector and the mapping parameter, the private vector of each group member is mapped to a new vector in the vector space according to the mapping parameter, and the group key is obtained by calculating the inner product of the new vector and the central vector.

Preferably, the step 1 is implemented by the following:

the group controller selects a mapping f and a finite field F for use by a group; each member selects a m-dimensional private vector over the finite field F, and sends the m-dimensional private vector to the group controller via secure channel; wherein m is a positive integer, and 2≦m≦n+1;

the group controller assigns and sends a serial number U_(i) to each member, and receives the m-dimensional private vector V_(i)=(v_(i,0),v_(i,1), . . . , v_(i,m−1)) of each member and keeps secret, wherein i=1, . . . , n;

the step 2 is implemented by the following:

the group controller selects random numbers v_(0,0),v_(0,1), . . . , v_(0,m−1) in the finite field F, and constructs a m-dimensional private vector of itself V₀=(v_(0,0),v_(0,1), . . . , v_(0,m−1)); the group controller selects a mapping parameter r in the finite field F, and maps private vectors V_(i)=(v_(i,0),v_(i,1), . . . , v_(i,m−1)) of all the members u_(i) and the m-dimensional private vector of itself V₀=(v_(0,0),v_(0,), . . . , v_(0,m−1)) into a new set of vectors via the mapping f according to the mapping parameter r:

for the private vector of the group controller itself V₀=(v_(0,0),v_(0,1), . . . , v_(0,m−1)), the group controller computes:

x_(0, 0) = f(v_(0, 0), r) x_(0, 1) = f(v_(0, 1), r) … x_(0, m − 1) = f(v_(0, m − 1), r)

for the private vectors V_(i)=(v_(i,0),v_(i,1), . . . , v_(i,m−1)) of the member u_(i), where i=1,2, . . . , m−1, the group controller computes:

x_(i, 0) = f(v_(i, 0), r) x_(i, 1) = f(v_(i, 1), r) … x_(i, m − 1) = f(v_(i, m − 1), r)

for the private vectors V_(i)=(v_(i,0),v_(i,1), . . . , v_(i,m−1)) of the member u_(i), where i=m,m+1, . . . , n, the group controller computes:

x_(i, 0) = f(v_(i, 0), r) x_(i, i − m + 2) = f(v_(i, 1), r) … x_(i.i) = f(v_(i, m − 1), r)

in this way, the group controller obtains a new set of vectors over the finite field F:

the new vector X₀ obtained after the mapping of the private vector of the group controller is:

X ₀=(x _(0,0) ,x _(0,1) , . . . , x _(0,n)), wherein x _(0,m) , . . . , x _(0,n) are all 0;

for the new vector X_(i) of the group member u_(i), where i=1,2, . . . , m−1;

X _(i)=(x _(i,0) ,x _(i,1) , . . . , x _(i,n)), wherein x _(i,m) , . . . , x _(i,n) are all 0;

for the new vector X_(i) of the group member u_(i), where i=m,m+1, . . . , n:

X _(i)=(x _(i,0) ,x _(i,1) , . . . , x _(i,n)), wherein x _(i,1) , . . . , x _(i,i−m+1) and x _(i,i+1) , . . . , x _(i,n) are all 0;

the group controller judges whether X₀,X₁ . . . , X_(n) are linearly independent, if they are linearly independent, then proceed to the step 3; otherwise, return to the step 2, or return to the step 1 to allow the group members to reselect private vectors; (since r is a random number, it is easily to obtain a set of vectors X₀,X₁ . . . , X_(n) that are linearly independent);

the step 3 is implemented by the following:

the group controller selects a random number K in the finite field F as a group key, and constructs a system of linear equations by using the new set of vectors and the group key:

suppose a₁,a₂, . . . , a_(n) are unknown parameters, and the group controller solves the central vector A=(a₀,a₁, . . . , a_(n)) from the following system of linear equations:

$\left\{ {\begin{matrix} {{{x_{0,0}a_{0}} + {x_{0,1}a_{1}} + \ldots + {x_{0,n}a_{n}}} = k} \\ {{{x_{1,0}a_{0}} + {x_{1,1}a_{1}} + \ldots + {x_{1,n}a_{n}}} = k} \\ \ldots \\ {{{x_{n,0}a_{0}} + {x_{n,1}a_{1}} + \ldots + {x_{n,n}a_{n}}} = k} \end{matrix}\quad} \right.$

this system of linear equations is represented in vector form: X×A^(T)=K^(T)

wherein the vector K=(k,k, . . . , k), the vector A=(a₀,a_(h1), . . . , a_(n)), the matrix

${X = {\begin{bmatrix} X_{0} \\ X_{1} \\ \ldots \\ X_{n} \end{bmatrix} = \begin{bmatrix} x_{0,0} & x_{0,1} & \ldots & x_{0,n} \\ x_{1,0} & x_{1,1} & \ldots & x_{1,n} \\ \ldots & \ldots & \ldots & \ldots \\ x_{n,0} & x_{n,1} & \ldots & x_{n,n} \end{bmatrix}}};$

since X₀,X₁ . . . , X_(n) are linearly independent vectors and the determinant of the coefficient matrix |X|≠0, this linear system of linear equations has the unique solution;

the central vector A=(a₀,a₁, . . . , a_(n)) and the mapping parameter r are broadcasted or multicasted by the group controller to all the group members via open channel;

the step 4 is implemented by the following:

after the group members receive the central vector A=(a₀,a₁ . . . , a_(n)) and the mapping parameter r, the private vector of the group member is mapped to a new vector in the vector space according to the mapping parameter r:

for the group member u_(i), where i=1,2, . . . , m−1:

x_(i, 0) = f(v_(i, 0), r) x_(i, 1) = f(v_(i, 1), r) … x_(i, m − 1) = f(v_(i, m − 1), r)

that is to say:

X _(i)=(x _(i,0) ,x _(i,1) , . . . , x _(i,n)), wherein x _(i,m) , . . . , x _(i,n) are all 0;

-   -   for the group member u_(i), where i=m,m+1, . . . , n:

x_(i, 0) = f(v_(i, 0), r) x_(i, i − m + 2) = f(v_(i, 1), r) … x_(i, i) = f(v_(i, m − 1), r)

-   -   that is to say:

X _(i)=(x _(i,0) ,x _(i1) , . . . x _(i,n)), wherein x _(i,1) , . . . , x _(i,i−m+1) and x _(i,i+1) , . . . , x _(i,n) are all 0;

then the group member u_(i) can calculate the group key k via the equation below, i.e. the inner product of the vector X_(i) and the central vector A=(a₀, a₁, . . . , a_(n))

k=X _(i) ×A ^(T) =x _(i,0) a ₀ +x _(i,1) a ₁ + . . . +x _(i,n) a _(n) wherein T is the transpose of the matrix;

when new members join the group, the group key management approach based on linear geometry further includes:

step 5, when new members join in the group, each new member selects a m-dimensional private vector in the finite field F, and sends the m-dimensional private vector to the group controller via secure channel; the group controller assigns and sends a serial number to each new member, and receives the m-dimensional private vector of each new member and keeps secret;

repeat the steps 2 to 4.

When group members need to leave the group, the group key management approach based on linear geometry further includes:

step 5, when group members need to leave the group, each member that needs to leave the group applies to the group controller for leaving the group; the group controller deletes the private vectors of the leaving members, and reassigns serial numbers for the remaining members according to the size order of the subscripts of the current members, and sends the serial numbers to all members via open channel;

repeat the steps 2 to 4.

When there are new members want to join in the group and members want to leave the group simultaneously, the group key management approach based on linear geometry further includes:

step 5, when new members want to join the group and members want to leave simultaneously, each new member selects a m-dimensional vector in the finite field F and sends the m-dimensional vector to the group controller via secure channel; the members that want to leave the group apply to the group controller for leaving the group, and then the group controller deletes the private vector of each leaving member and reassigns serial numbers to the remaining members according to the size order of the subscripts of the current members, and assigns a serial number to each new member; the m-dimensional vector of the new member is received by the group controller and kept secret; then the subscripts of all members are broadcasted to all members via open channel;

repeat the steps 2 to 4.

Preferably, the m-dimensional private vector is a two-dimensional private vector.

Preferably, the group key management approach based on linear geometry further includes auto update: if no member joins or leaves the group in a preset period, then the group controller will update the group key periodically, the group controller reselects the mapping parameter and the group key and calculates the central vector, and the central vector and the mapping parameter are broadcasted or multicasted to all members by the group controller via open channel.

Preferably, suppose the mapping f can be represented by z=f(w,y), where w,y,zεF, the main function of the mapping f is randomization, and the mapping f conforms to the following characteristics:

1) it is easy to calculate z=f(w,y) if w, y are known;

2) it is difficult to compute w if only z and y are known; if z and w are known, it is also difficult to compute y from z=f (w,y); it is difficult to compute w_(i) from z_(i)=f(w_(i),y_(i)), though a series of z_(i) and y_(i) is obtained; and it is also difficult to compute y_(i) from z_(i)=f(w_(i),y_(i)), though a series of z, and w, is obtained.

Comparing with the prior art, the present invention has the following advantages:

Firstly, the storage of each member and the computation cost of the group controller are reduced. In the preferred scheme, if we fix m=2, each member needs to store only two-dimensional private vector of itself, that is to say, only 2 L bit storage space is required (L is the bit size of each element in the finite field F), and the computation cost of each member consists of mapping the private vector of itself to a new vector in the space according to the mapping parameter and computing the group key, and this includes two mapping operations, two multiplications and one addition over the finite field. It can be seen that the storage space and computation cost of each member is fixed, and will not increase as the group size increases. Moreover, the group controller needs to store the private vectors of the entire group, the storage space required is 2 nL (n is the quantity of the group members, and L is the bit size of each element in the finite field), and the main computation of the group controller is to compute the central vector; because the coefficient matrix is sparse and can be easily converted into a triangle matrix, the computation is very easy accordingly. Therefore, the computation cost and the storage space of the group controller increases linearly only as the group grows.

Secondly, the computation of the group controller can be easily parallelized. If the group controller operates on a multi-core processor platform, then it is very easy to enable the computation of the group controller to be parallel by using the current popular parallel computing library, which takes the advantage of the multi-core processor.

Thirdly, the secure channel is required only when a group member registers for the first time and a new member joins the group, and in other situations, only the open channel is required. This is because during the initialization of the group, the private vector is sent by the group member to the group controller via the secure channel, and at this time, the group has not yet been established, the secure channel is needed in order to ensure the security of the private vector. After then, the group controller only needs to send the central vector A and the mapping parameter r to all the group members, and because the vector A and r are both public, it is not necessary to keep secret, the secure channel is not required, and the open channel can be used for broadcasting and multicasting.

Fourthly, the method of the present invention is independent of other cryptography methods. The security of the present invention is based on the linear geometry theory over the finite field, only simple mapping operation and basic operation over the finite field is used during the process of computing the group key, and it does not rely on other traditional cryptography methods including asymmetric cryptography, symmetric cryptography, and hash function. In this way, the possibility that the present invention is attacked by other aspects is reduced. Even if the traditional cryptography methods are broken, the security provided by the present invention will not be affected.

Fifthly, forward and backward secrecy is provided. The group key k is randomly selected, and k will be changed each time the group members join or leave. Even if the group key is exposed for a period of time, non-members will not know the group key for the next time period. Therefore, the forward and backward secrecy can be guaranteed.

Sixthly, the attack from the group members or non-members can be effectively avoided. The group key k is obtained by computing the inner product of the private vector X_(i) of the group member and the central vector A published by the group, while X_(i) is generated by the mapping f by using the private vector V_(i) of the group member and the mapping parameter r. For any non-member, X_(i) is unable to be computed if the private vector V_(i) is illegitimate, and thus the group key k can not be obtained. Any group member or non-member can not derive the private vector of other group members, since the private vector of each group member is sent to the group controller via the secure channel. Therefore, any non-member or other group members can not derive the private vector of the group member.

Finally, brute force attack to explore the group key is extremely difficult: the group key k is randomly selected from the finite field F. As long as the number of elements in the field is greater than a certain constant, e.g. 2¹²⁸, then it will be very difficult to explore the group key by brute force attack.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a secure group communication system according to a first embodiment of the present invention;

FIG. 2 is a schematic diagram showing that the group controller selects the finite field and the mapping f for the group use, and each group member sends the private vector to the group controller, according to the first embodiment;

FIG. 3 is a schematic diagram showing that the group controller computes the central vector through the mapping parameter and the group key, according to the first embodiment;

FIG. 4 is a schematic diagram showing that the group controller sends the mapping parameter and the central vector to the group members, according to the first embodiment;

FIG. 5 is a schematic diagram showing that the group members compute the mapping parameter and the central vector, according to the first embodiment;

FIG. 6 is a schematic diagram showing that the group controller and the group members form the group, according to the first embodiment;

FIG. 7 is a schematic diagram showing a secure group communication system according to a fifth embodiment of the present invention;

FIG. 8 is a schematic diagram showing that the group controller selects the finite field and the mapping f for the group use, and each group member sends the private vector to the group controller, according to the fifth embodiment;

FIG. 9 is a schematic diagram showing that the group controller computes the central vector through the mapping parameter and the group key, according to the fifth embodiment;

FIG. 10 is a schematic diagram showing that the group controller sends the mapping parameter and the central vector to the group members, according to the fifth embodiment;

FIG. 11 is a schematic diagram showing that the group members compute the mapping parameter and the central vector, according to the fifth embodiment;

FIG. 12 is a schematic diagram showing that the group controller and the group members form the group, according to the fifth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be further described in detail in the following embodiments accompanying the drawings. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended.

Embodiment 1

Referring to FIG. 1, a typical secure group communication system includes a group controller (GC) and four group users U1, U2, U3 and U4. The group controller is connected to each user via interne.

As shown in FIG. 2, during the initialization of the group, the group controller selects a mapping f and a finite field F for use by the group, all computations in the group are performed over the finite field, and the mapping f uses a pseudo random number generator.

step 1, group members U1, U2, U3, U4 want to join the group, the group member U1 selects three private random numbers v_(1,1),v_(1,2),v_(1,3)εF in the finite field F and constructs the₃ three-dimensional vector V₁=(v_(1,1),v_(1,2),v_(1,3)), the group member U2 also selects three private random numbers v_(2,1),v_(2,2,),v_(2,3)εF and constructs the three-dimensional vector V₂=(v_(2,1),v_(2,2),v_(2,3)), the group member U3 also selects three private random numbers v_(3,1),v_(3,2),v_(3,3)εF and constructs the three-dimensional vector V₃=(v_(3,1),v_(3,2),v_(3,3)), and the group member U4 also selects three private random numbers v_(4,1),v_(4,2),v_(4,3)εF and constructs the three-dimensional vector V₄=(v_(4,1),v_(4,2),v_(4,3)). The group members U1, U2, U3 and U4 send the vectors V₁=(v_(1,1),v_(1,2),v_(1,3)), V₂=(v_(2,1),v_(2,2),v_(2,3)). V₃=(v_(3,1),v_(3,2),v_(3,3)) and V₄=(v_(4,1),v_(4,2),v_(4,3)) to the group controller.

The group controller assigns the serial number u₁ to the group member U1, the serial number u₂ to the group member U2, the serial number u₃ to the group member U3, and the serial number u₄ to the group member U4. The group controller sends these serial numbers to the corresponding group members via secure channel, and receives the three-dimensional private vector V_(i)=(v_(i,1),v_(i,2),v_(i,3)) of each group member and keeps secret, wherein i=1,2,3,4.

Step 2, as shown in FIG. 3, the group controller (GC) receives the private vectors V₁, V₂, V₃, V₄ of the group members U1, U2, U3 and U4, and selects a random number r in the finite field F as the mapping parameter, and maps the three-dimensional private vectors V_(i)=(v_(i,1),v_(i,2),v_(i,3)) of all the members u_(i) into a new set of vectors via the mapping f:

for each member u_(i) (i=1,2,3), the group controller computes:

x _(1,1) =f(v _(1,1) ,r)

x _(1,2) =f(v _(1,2) ,r)

x _(1,3) =f(v _(1,3) ,r)

x _(2,1) =f(v _(2,1) ,r)

x _(2,2) =f(v _(2,2) ,r)

x _(2,3) =f(v _(2,3) ,r)

x _(3,1) =f(v _(3,1) ,r)

x _(3,2) =f(v _(3,2) ,r)

x _(3,3) =f(v _(3,3) ,r)

for each member u_(i), where i=4, the group controller computes:

x ₄₁ =f(v _(4,1) ,r)

x _(4,3) =f(v _(4,2) ,r)

x _(4,4) =f(v _(4,3) ,r)

in this way, the group controller obtains a new set of vectors over the finite field F:

for the new vector X₁ of the group member u_(i) (where i=1, 2, 3):

X _(i)=(x _(i,1) ,x _(i,2) ,x _(i,3),0), wherein x_(i,4) is 0;

for the new vector X₄ of the group member u₄:

X ₄=(x _(4,1),0,x _(4,3) ,x _(4,4))

The group controller judges whether X₁,X₂,X₃,X₄ are linearly independent, and if they are independent, then proceed to the step 3, otherwise, return to the step 2. Since r is a random number, it is easy to obtain a set of vectors X₁,X₂,X₃,X₄ that are linearly independent.

Step 3, the group controller selects a random number kεF as a group key, and constructs a system of linear equations by using the new set of vectors and the group key:

suppose a₁,a₂,a₃,a₄ are unknown parameters, and the group controller computes the central vector A=(a₁,a₂,a₃,a₄) from the following system of linear equations:

$\left\{ {\begin{matrix} {{{x_{1,1}a_{1}} + {x_{1,2}a_{2}} + {x_{1,3}a_{3}}} = k} \\ {{{x_{2,1}a_{1}} + {x_{2,2}a_{2}} + {x_{2,3}a_{3}}} = k} \\ {{{x_{3,1}a_{1}} + {x_{3,2}a_{2}} + {x_{3,3}a_{3}}} = k} \\ {{{x_{4,1}a_{1}} + {x_{4,3}a_{3}} + {x_{4,4}a_{4}}} = k} \end{matrix}\quad} \right.$

this system of linear equations is represented in vector form: X×A^(T)=K^(T)

wherein the vector K=(k,k,k,k), A=(a₁,a₂,a₃,a₄), the matrix

$X = {\begin{bmatrix} X_{1} \\ X_{2} \\ X_{3} \\ X_{n} \end{bmatrix} = \begin{bmatrix} x_{1,1} & x_{1,2} & x_{1,3} & 0 \\ x_{2,1} & x_{2,2} & x_{2,3} & 0 \\ x_{3,1} & x_{3,2} & x_{3,3} & 0 \\ x_{4,1} & 0 & x_{4,3} & x_{4,4} \end{bmatrix}}$

since the determinant of the coefficient matrix |X|≠0, this linear system of linear equations has the unique solution A=(a₁,a₂,a₃,a₄), and A=(a₁,a₂,a₃,a₄) is the central vector.

As shown in FIG. 4, the group controller broadcasts the central vector A=(a₁,a₂,a₃,a₄) and the mapping parameter r to the group members U1, U2, U3, U4 via open channel.

Step 4, as shown in FIG. 5, after the group members U1, U2, U3 and U4 receive the central vector A=(a₁,a₂,a₃,a₄) and the mapping parameter r, a new vector is computed according to the mapping parameter r:

for the group member u_(i) (i=1, 2, 3):

x _(1,1) =f(v _(1,1) ,r)

x _(1,2) =f(v _(1,2) ,r)

x _(1,3) =f(v _(1,3) ,r)

x _(2,1) =f(v _(2,1) ,r)

x _(2,2) =f(v _(2,2) ,r)

x _(2,3) =f(v _(2,3) ,r)

x _(3,1) =f(v _(3,1) ,r)

x _(3,2) =f(v _(3,2) ,r)

x _(3,3) =f(v _(3,3) ,r)

for the group member u_(i), where i=4:

x _(4,1) =f(v _(4,1) ,r)

x _(4,3) =f(v _(4,2) ,r)

x _(4,4) =f(v _(4,3) ,r)

then, U1 calculates the group key k=x_(1,1)a₁+x_(1,2)a₂+x_(1,3)a₃, U2 calculates the group key k=x_(2,1)a₁+x_(2,2)a₂+x_(2,3)a₃, U3 calculates the group key k=x_(3,1)a₁+x_(3,2)a₂+x_(3,3)a₃, U4 calculates the group key k=x_(4,1)a₁+x_(4,3)a₃+x_(4,4)a₄, the calculated result k of the U1 is identical to the calculated results k of the U2, U3 and U4.

As shown in FIG. 6, after the above steps, a group with the group members U1, U2, U3 and U4 is established.

If no member joins or leaves the group in a preset period, then the group controller will update the group key periodically, i.e. repeat the step 2 to step 4.

Embodiment 2

Suppose the secure group communication system includes a group controller (GC) and five group users U1, U2, U3, U4 and U5. The group controller is connected to each user via internet. During the initialization of the group, the group controller selects a mapping f and a finite field F for use by the group, all computations in the group are performed over the finite field, and the mapping f uses a pseudo random number generator.

Step 1, the group member U4 applies to the group controller for leaving the group;

the group controller deletes the private vector V₄ of the leaving member, and reassigns serial numbers u_(i) for the remaining members according to the size order of the subscripts of the current members, wherein i=1,2,3,4; the group controller sends the serial numbers to all members via open channel, and the private vectors of the group members currently stored by the group controller are V₁,V₂,V₃,V₄;

Other steps are identical to the steps 2 to 4 of the Embodiment 1.

Embodiment 3

Suppose the secure group communication system includes a group controller (GC) and two group users U1 and U2. The group controller is connected to each user via internet. During the initialization of the group, the group controller selects a mapping f and a finite field F for use by the group, all computations in the group are performed over the finite field, and the mapping f uses a pseudo random number generator.

Step 1, group members U3, U4 want to join the group, the group member U3 selects three private random numbers v_(3,1),v_(3,2),v_(3,3)εF and constructs the three-dimensional vector V₃=(v_(3,1),v_(3,2),v_(3,3)), and sends the vector V₃=(v_(3,1),v_(3,2),v_(3,3)) to the group controller via secure channel; the group member U4 also selects three private random numbers v_(4,1),v_(4,2),v_(4,3)εF and constructs the three-dimensional vector V₄=(v_(4,1),v_(4,2),v_(4,3)), and sends the vector V₄=(v_(4,1),v_(4,2),v_(4,3)) to the group controller via secure channel.

The group controller assigns the serial number u₃ to the group member U3, and sends the serial number u₃ to this group member, and receives the private vector V₃=(v_(3,1),v_(3,2),v_(3,3)) of this group member and keeps secret; the group controller assigns the serial number u₄ to the group member U4, and sends the serial number u₄ to this group member, and receives the private vector V₄=(v_(4,1),v_(4,2),v_(4,3)) of this group member and keeps secret; therefore, the private vectors currently stored by the group controller are V₁,V₂,V₃,V₄.

Other steps are identical to the steps 2 to 4 of the Embodiment 1.

Embodiment 4

Suppose the secure group communication system includes a group controller (GC) and three group users U1, U2 and U3. The group controller is connected to each user via interne.

During the initialization of the group, the group controller selects a mapping f and a finite field F for use by the group, all computations in the group are performed over the finite field, and the mapping f uses a pseudo random number generator.

Step 1, if the group member U2 wants to leave the group and the new group members U4 and U5 want to join the group, the new group members U4 and U5 select a three-dimensional vector respectively over the finite field F, and send the vectors to the group controller via secure channel.

The group controller deletes the private vector V₂ of the leaving member U2, and reassigns a subscript serial number i to allow the original group member U3 to be changed into u₂, and the corresponding vector is V₂=(v_(2,1),v_(2,2),v_(2,3)). The group controller assigns serial numbers u₃ and u₄ respectively to the new members U4 and U5, in this way, the U4 becomes u₃ in the group and the corresponding vector is V₃=(v_(3,1),v_(3,2),v_(3,3)), while the U5 becomes u₄ in the group and the corresponding vector is V₄=(v_(4,1),v_(4,2),v_(4,3)). The group controller sends the serial numbers to all members via open channel, and the private vectors of the group members currently stored by the group controller are V₁, V₂, V₃, V₄.

Other steps are identical to the steps 2 to 4 of the Embodiment 1.

Embodiment 5

Referring to FIG. 7, a typical secure group communication system includes a group controller (GC) and three group users U1, U2 and U3. The group controller is connected to each user via internet.

As shown in FIG. 8, during the initialization of the group, the group controller selects a mapping f and a finite field F for use by the group, all computations in the group are performed over the finite field, and the mapping f uses a pseudo random number generator.

Step 1, group members U1, U3, U4 want to join the group, the group member U1 selects two private random numbers v_(1,0),v_(1,1)εF and constructs the two-dimensional vector V₁=(v_(1,0),v_(1,1)); the group member U2 also selects two private random numbers v_(2,0),v_(2,1)εF and constructs the two-dimensional vector V₂=(v_(2,0),v_(2,1)); and the group member U3 also selects two private random numbers v_(3,0),v_(3,1)εF and constructs the two-dimensional vector V₃=(v_(3,0),v_(3,1)). The U1, U2 and U3 send the vectors V₁=(v_(1,0),v_(1,1)), V₂=(v_(2,0),v_(2,1)), and V₃=(v_(3,0),v_(3,1)) to the group controller via secure channel.

The group controller assigns the serial number u₁ to the group member U1, the serial number u₂ to the group member U2, and the serial number u₃ to the group member U3; the group controller sends the above-mentioned serial numbers to the corresponding group members via secure channel, and receives the two-dimensional private vector V_(i)=(v_(i,0),v_(i,1)) of each group member and keeps secret, wherein i=1,2,3;

Step 2, as shown in FIG. 9, after receiving the private vectors V₁, V₂, V₃ of the group members U1, U2 and U3, the group controller selects random numbers v_(0,0),v_(0,1), rεF in the finite field F, wherein r is the mapping parameter, v_(0,0),v_(0,1) construct the private vector of the group controller V₀=(v_(0,0),v_(0,1)) and V₀ is stored by the group controller.

The group controller maps the private vector of itself and the private vectors of all group members according to the mapping parameter to form a new set of vectors:

for the private vector V₀=(v_(0,0),v_(0,1)) of the group controller itself, the group controller computes:

x _(0,0) =f(v _(0,0) ,r)

x _(0,1) =f(v _(0,1) ,r)

for the group member u_(i) (i=1), the group controller computes:

x _(1,0) =f(v _(1,0) ,r)

x _(1,1) f=(v _(1,1) ,r)

the group controller judges whether x_(0,0)x_(1,1)−x_(0,1)x_(1,o)=0 is established, and if yes, then return to reselect the random numbers v_(0,0), v_(0,1), rεF; if not, then proceed to the next step:

for the group member u_(i) (where i=2, 3), the group controller computes:

x _(2,0) =f(v _(2,0) ,r)

x _(2,2) =f(v _(2,1) ,r)

x _(3,0) =f(v _(3,0) ,r)

x _(3,3) =f(v _(3,1) ,r)

in this way, the group controller obtains a new set of vectors over the finite vector F:

for the new vector X₀ obtained after the mapping of the private vector of the group controller:

X ₀=(x _(0,0) ,x _(0,1),0,0)

for the new vector X₁ of the group member u₁:

X ₁=(x _(1,0) ,x _(1,1),0,0)

for the new vector X_(i) of the group member u_(i), where i=2,3:

X ₂=(x _(2,0),0,x _(2,2),0)

X ₃=(x _(3,0),0,0,x _(3,3))

The group controller judges whether X₀, X₁, X₂, X₃ are linearly independent, and computes |X|=(x_(0,0)x_(1,1)−x_(0,1)x_(1,0))x_(2,2)x_(3,3), and if it is not zero, then X₀, X₁, X₂, X₃ are linearly independent. If they are linearly dependent, then reselect the random numbers v_(0,0),v_(0,1), rεF and compute X₀, X₁, X₂, X₃, otherwise, proceed to the next step. Since it meets the requirement of x_(0,0)x_(1,1)−x_(0,1)x_(1,0)≠0, as long as x_(i,i)≠0(i=2,3), then |X|0. Therefore, it is easy to obtain a set of vectors X₀, X₁, X₂, X₃ that are linearly independent according to the random number r and the mapping f.

Step 3, the group controller selects a random number kεF as the group key. Suppose a₀,a₁, . . . , a_(n) are unknown variables, the group controller computes the central vector A=(a₀,a₁, . . . , a_(n)) from the following system of linear equations:

$\left\{ {\begin{matrix} {{{x_{0,0}a_{0}} + {x_{0,1}a_{1}}} = k} \\ {{{x_{1,0}a_{0}} + {x_{1,1}a_{1}}} = k} \\ {{{x_{2,0}a_{0}} + {x_{2,2}a_{2}}} = k} \\ {{{x_{3,0}a_{0}} + {x_{3,3}a_{3}}} = k} \end{matrix}\quad} \right.$

this system of linear equations can be represented in vector form: X×A^(T)=K^(T)

wherein the vector K=(k,k,k,k), the vector A=(a₀,a₁,a₂,a₃), the matrix

$X = {\begin{bmatrix} X_{0} \\ X_{1} \\ X_{2} \\ X_{3} \end{bmatrix} = \begin{bmatrix} x_{0,0} & x_{0,1} & 0 & 0 \\ x_{1,0} & x_{1,1} & 0 & 0 \\ 0 & 0 & x_{2,2} & 0 \\ 0 & 0 & 0 & x_{3,3} \end{bmatrix}}$

Because the determinant of the coefficient matrix X|≠0, this linear system of linear equations has the unique solution A=(a₀,a₁,a₂,a₃), and A=(a₀,a₁,a₂,a₃) is the central vector.

As shown in FIG. 10, the group controller broadcasts the central vector A=(a₀,a₁,a₂,a₃) and the mapping parameter r to the group members U1, U2 and U3 via open channel.

Step 4, as shown in FIG. 11, after the group members U1, U2 and U3 receive A=(a₀,a₁,a₂,a₃) and the mapping parameter r, new vectors are computed according to the mapping parameter r:

x _(1,0) =f(v _(1,0) ,r)

x _(1,1) =f(v _(1,1) ,r)

x _(2,0) =f(v _(2,0) ,r)

x _(2,2) =f(v _(2,1) ,r)

x _(3,0) =f(v _(3,0) ,r)

x _(3,3) =f(v _(3,1) ,r)

then, the group member U1 computes the group key k=x_(1,0)a₀+x_(1,1)a₁, the group member U2 computes the group key k=x_(2,0)a₀+x_(2,2)a₂, the group member U3 computes the group key k=x_(3,0)a₀+x_(3,3)a₃. It is obvious that the group key k calculated by the group member U1 is identical to the group keys k calculated by the group members U2 and U3.

As shown in FIG. 12, a group with group members U1, U2 and U3 is established after the above steps.

If no member joins or leaves the group in a preset period, then the group controller will update the group key periodically, that is to say, repeat the steps 2 to 4.

Embodiment 6

Suppose the secure group communication system includes a group controller (GC) and four group users U1, U2, U3 and U4. The group controller is connected to each user via interne. During the initialization of the group, the group controller selects a mapping f and a finite field F for use by the group, all computations in the group are performed over the finite field, and the mapping f uses a pseudo random number generator.

Step 1, if the group member U4 applies to the group controller for leaving the group;

the group controller deletes the private vector V₄ of the leaving member, and reassigns serial number u_(i) for the remaining members according to the size order of the subscripts of the current members, wherein i=1,2,3; the group controller sends the serial numbers to all members via open channel, and the private vectors of the group members currently stored by the group controller are V₁,V₂,V₃;

Other steps are identical to the steps 2 to 4 of the Embodiment 5.

Embodiment 7

Suppose the secure group communication system includes a group controller (GC) and two group users U1 and U2. The group controller is connected to each user via internet. During the initialization of the group, the group controller selects a mapping f and a finite field F for use by the group, all computations in the group are performed over the finite field, and the mapping f uses a pseudo random number generator.

Step 1, when a new group member U3 wants to join the group, the new group member U3 selects a private vector over the finite field F, and sends it to the group controller via secure channel

The group controller assigns a serial number u₃ to the new group member U3 and sends the serial number to this group member, and then receives the private vector V₃=(v_(3,0),v_(3,1)) of this group member and keeps secret. Therefore, the private vectors currently stored by the group controller store are V₁,V₂,V₃.

Other steps are identical to the steps 2 to 4 of the Embodiment 1.

Embodiment 8

Suppose the secure group communication system includes a group controller (GC) and three group users U1, U2 and U3. The group controller is connected to each user via internet.

During the initialization of the group, the group controller selects a mapping f and a finite field F for use by the group, all computations in the group are performed over the finite field, and the mapping f uses a pseudo random number generator.

Step 1, if the group member U2 wants to leave the group and a new group member U4 wants to join the group, the new group member U4 selects a two-dimensional vector over the finite field F, and sends the vector to the group controller via secure channel.

The group controller deletes the private vector V₂ of the leaving member U2, and reassigns a subscript serial number i to allow the original group member U3 to change into u₂, and the corresponding vector is V₂=(v_(2,0),v_(2,)). The group controller assigns serial numbers u₃ to the new member U4, in this way, the U4 becomes u₃ in the group and the corresponding vector is V₃=(v_(3,0),v_(3,1)). The group controller sends the serial numbers to all members via open channel, and the private vectors of the group members currently stored by the group controller are V₁,V₂,V₃.

Other steps are identical to the steps 2 to 4 of the Embodiment 5.

It should be emphasized that the above-described embodiments can be combined freely. Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and the present invention and protected by the following claims. 

1. A group key management approach based on linear geometry, comprising the following steps: step 1: a group controller selects a mapping f and a finite field F for use by a group; suppose the group has n group members, each group member selects a m-dimensional private vector over the finite field F, and sends the m-dimensional private vector to the group controller via secure channel; the group controller assigns and sends a serial number to each group member, and receives the m-dimensional private vector of each group member and keeps secret; wherein m, n are positive integers, and 2≦m≦n; step 2: the group controller selects a mapping parameter in the finite field F randomly, and maps the private vectors of all the group members into a new set of vectors in a vector space by using the mapping f according to the mapping parameter, if the set of vectors is linearly dependent, then the mapping parameter is reselected to perform remapping, or return to the step 1 to allow each group member to reselect a private vector, until the new set of vectors is linearly independent; step 3: the group controller selects a random number in the finite field F as a group key, and constructs a system of linear equations by using the new set of vectors and the group key; the group controller computes the unique solution of the system of linear equations which is called central vector, and the central vector and the mapping parameter are broadcasted or multicasted by the group controller to all the group members via open channel; step 4: after the group members receive the central vector and the mapping parameter, the private vector of each group member is mapped to a new vector in a vector space according to the mapping parameter, and the group key is obtained by calculating the inner product of the new vector and the central vector.
 2. The group key management approach based on linear geometry of claim 1, further comprising: step 5: when new group members join in the group, each new group member selects a m-dimensional private vector in the finite field F, and sends the m-dimensional private vector to the group controller via secure channel; the group controller assigns and sends a serial number to each new group member, and receives the m-dimensional private vector of each new group member and keeps secret; repeat the steps 2 to
 4. 3. The group key management approach based on linear geometry of claim 1, further comprising: step 5: when group members need to leave the group, each group member that needs to leave the group applies to the group controller for leaving the group; the group controller deletes the private vectors of the leaving group members, and reassigns serial numbers for the remaining group members according to the size order of the subscripts of the current group members, and sends the serial numbers to all group members via open channel; repeat the steps 2 to
 4. 4. The group key management approach based on linear geometry of claim 1, wherein the step 1 is implemented by the following: the group controller selects a mapping f and a finite field F for use by a group; each group member selects a m-dimensional private vector over the finite field F, and sends the m-dimensional private vector to the group controller via secure channel; wherein m is a positive integer, and 2≦m≦n; the group controller assigns and sends a serial number u_(i) to each group member, and receives the m-dimensional private vector V_(i)=(v_(i,1),v_(i,2), . . . , v_(i,m)) of each group member and keeps secret, wherein i=1, . . . , n; the step 2 is implemented by the following: the group controller selects a mapping parameter r in the finite field F randomly, and maps private vectors V_(i)=(v_(i,1),v_(i,2), . . . , v_(i,m)) of all the group members u_(i) into a new set of vectors via the mapping f according to the mapping parameter r: for each group member u_(i), where i=1,2, . . . , m: x_(i, 1) = f(v_(i, 1), r) x_(i, 2) = f(v_(i, 2), r) … x_(i, m) = f(v_(i, m), r) for each group member u_(i), where i=m+1, . . . , n: x_(i, 1) = f(v_(i, 1), r) x_(i, i − m + 2) = f(v_(i, 2), r) … x_(i, i) = f(v_(i, m), r) in this way, the group controller obtains a new set of vectors in the finite field F: for each group member u_(i), where i=1,2, . . . , m: X _(i)=(x_(i,1) ,x _(i,2) , . . . ,x _(i,n)), wherein x_(i,m+1), . . . , x_(i,n) are all 0; for each group member u_(i), where i=m+1, . . . , n: X _(i)(x _(i,1) ,x _(i,2) , . . . ,x _(i,n)), wherein x_(i,2), . . . ,x_(i,i−m+1) and x_(i,i+1), . . . ,x_(i,n) are all 0; the group controller judges whether X₁,X₂ . . . , X_(n) are linearly independent, and if they are linearly independent, then proceed to the step 3; otherwise, return to the step 2, or return to the step 1 to allow each group member to reselect a private vector, the step 3 is implemented by the following: the group controller selects a random number K in the finite field F as a group key, and constructs a system of linear equations by using the new set of vectors and the group key: suppose a₁,a₂, . . . , a_(n) are unknown parameters, and the group controller computes the central vector A=(a₁,a₂, . . . , a_(n)) from the following system of linear equations: $\left\{ {\begin{matrix} {{{x_{1,1}a_{1}} + {x_{1,2}a_{2}} + \ldots + {x_{1,n}a_{n}}} = k} \\ {{{x_{2,1}a_{1}} + {x_{2,2}a_{2}} + \ldots + {x_{2,n}a_{n}}} = k} \\ \ldots \\ {{{x_{n,1}a_{1}} + {x_{n,2}a_{2}} + \ldots + {x_{n,n}a_{n}}} = k} \end{matrix}\quad} \right.$ this system of linear equations is represented in vector form: X×A^(T)=K^(T) wherein T represents the transpose of the matrix, the vector K=(k,k, . . . k), the vector A=(a₁,a₂, . . . , a_(n)), and the matrix ${X = {\begin{bmatrix} X_{1} \\ X_{2} \\ \ldots \\ X_{n} \end{bmatrix} = \begin{bmatrix} x_{1,1} & x_{1,2} & \ldots & x_{1,n} \\ x_{2,1} & x_{2,2} & \ldots & x_{2,n} \\ \ldots & \ldots & \ldots & \ldots \\ x_{n,1} & x_{n,2} & \ldots & x_{n,n} \end{bmatrix}}};$ since X₁,X₂, . . . , X_(n) are linearly independent vectors and the determinant of the coefficient matrix |X|≠0, this system of linear equations has the unique solution; the central vector A=(a₁,a₂, . . . , a_(n)) and the mapping parameter r are broadcasted or multicasted by the group controller to all the group members via open channel; the step 4 is implemented by the following: after the group members receive the central vector A=(a₁,a₂, . . . , a_(n)) and the mapping parameter r, the private vector of each group member u_(i) is mapped to a new vector in the vector space according to the mapping parameter r: for the group member u_(i), where i=1,2, . . . , m: x_(i, 1) = f(v_(i, 1), r) x_(i, 2) = f(v_(i, 2), r) … x_(i, m) = f(v_(i, m), r) that is to say: X _(i)=(x_(i,1) ,x _(i,2) , . . . ,x _(i,n)), wherein x_(i,m+1), . . . , x_(i,n) are all 0; for the group member u_(i), wherein i=m+1, . . . , n: x_(i, 1) = f(v_(i, 1), r) x_(i, i − m + 2) = f(v_(i, 2), r) … x_(i, i) = f(v_(i, m), r) that is to say: X _(i)=(x _(i,1) ,x _(i,2) , . . . ,x _(i,n)), wherein x _(i,2) , . . . ,x _(i,i−m+1) and x _(i,i+1) , . . . ,x _(i,n) are all 0; then the group member u_(i) can calculate the group key k via the equation below, i.e. the inner product of the vector X_(i) and the central vector A=(a₁,a₂, . . . , a_(n)) k=X _(i) ×A ^(T) =x _(i,1) a ₁ +x _(i,2) a ₂ + . . . +x _(i,n) a _(n), where T is the transpose of the matrix.
 5. A group key management approach based on linear geometry, comprising the following steps: step 1: a group controller selects a mapping f and a finite field F for use by a group; suppose the group has n group members, each group member selects a m-dimensional private vector over the finite field F, and sends the m-dimensional private vector to the group controller via secure channel; the group controller assigns and sends a serial number to each group member, and receives the m-dimensional private vector of each group member and keeps secret; wherein m, n are positive integers, and 2≦m≦n+1; step 2: after the group controller receives the private vectors of all group members, the group controller itself also selects a m-dimensional private vector, and the group controller selects a mapping parameter in the finite field F randomly, and maps the private vector of the group controller and the private vectors of all the group members into a new set of vectors via the mapping f according to the mapping parameter; if the set of vectors is linearly dependent, then the mapping parameter is reselected to perform remapping, or return to step 1 to make each group member reselect a private vector, until the new set of vectors is linearly independent; step 3: the group controller selects a random number in the finite field F as a group key, and constructs a system of linear equations by using the new set of vectors and the group key; the group controller computes the unique solution of the system of linear equations which is called central vector, and the central vector and the mapping parameter are broadcasted or multicasted by the group controller to all the group members via open channel; step 4: after the group members receive the central vector and the mapping parameter, the private vector of each group member is mapped to a new vector in the vector space according to the mapping parameter, and the group key is obtained by calculating the inner product of the new vector and the central vector.
 6. The group key management approach based on linear geometry of claim 5, further comprising: step 5: when new group members join in the group, each new group member selects a m-dimensional private vector in the finite field F, and sends the m-dimensional private vector to the group controller via secure channel; the group controller assigns and sends a serial number to each new group member, and receives the m-dimensional private vector of each new group member and keeps secret; repeat the steps 2 to
 4. 7. The group key management approach based on linear geometry of claim 5, further comprising: step 5, when group members need to leave the group, each group member that needs to leave the group applies to the group controller for leaving the group; the group controller deletes the private vectors of the leaving group members, and reassigns serial numbers for the remaining group members according to the size order of the subscripts of the current group members, and sends the serial numbers to all group members via open channel; repeat the steps 2 to
 4. 8. The group key management approach based on linear geometry of claim 5, wherein the step 1 is implemented by the following: the group controller selects a mapping f and a finite field F for use by a group; each group member selects a m-dimensional private vector over the finite field F, and sends the m-dimensional private vector to the group controller via secure channel; wherein m is a positive integer, and 2≦m≦n+1; the group controller assigns and sends a serial number u_(i) to each group member, and receives the m-dimensional private vector V_(i)=(v_(i,0),v_(i,1), . . . , v_(i,m−1)) of each group member and keeps secret, wherein i=1, . . . , n; the step 2 is implemented by the following: the group controller selects random numbers v_(0,0),v_(0,1), . . . , v_(0,m−1) in the finite field F, and constructs a m-dimensional private vector of itself V₀=(v_(0,0),v_(0,1), . . . , v_(0,m−1)); the group controller selects a mapping parameter r in the finite field F, and maps private vectors V_(i)=(v_(i,0),v_(i,1), . . . , v_(i,m−1)) of all the group members u_(i) and the m-dimensional private vector V₀=(v_(0,0),v_(0,1), . . . , v_(0,m−1)) into a new set of vectors via the mapping f according to the mapping parameter r: for the private vector of the group controller itself V₀=(v_(0,0),v_(0,1), . . . , v_(0,m−1)), the group controller computes: x_(0, 0) = f(v_(0, 0), r) x_(0, 1) = f(v_(0, 1), r) … x_(0, m − 1) = f(v_(0, m − 1), r) for the private vectors V_(i)=(v_(i,0),v_(i,1), . . . , v_(i,m−1)) of the group members u_(i), where i=1, 2, . . . , m−1, the group controller computes: x_(i, 0) = f(v_(i, 0), r) x_(i, 1) = f(v_(i, 1), r) … x_(i, m − 1) = f(v_(i, m − 1), r) for the private vectors V_(i)=(v_(i,0),v_(i,1), . . . , v_(i,m−1)) of the group members u_(i), where i=m,m+1, . . . , n, the group controller computes: x_(i, 0) = f(v_(i, 0), r) x_(i, i − m + 2) = f(v_(i, 1), r) … x_(i, i) = f(v_(i, m − 1), r) in this way, the group controller obtains a new set of vectors over the finite field F: the new vector X₀ obtained after the mapping of the private vector of the group controller is: X ₀=(x _(0,0) ,x _(0,1) , . . . ,x _(0,n)), wherein x _(0,m) , . . . ,x _(0,n) are all 0; for the new vector X_(i) of the group members u_(i), where i=1,2, . . . , m−1: X _(i)=(x _(i,0) ,x _(i,1) , . . . ,x _(i,n)), wherein x _(i,m) , . . . ,x _(in) are all 0; for the new vector X_(i) of the group member u_(i), where i=m,m+1, . . . , n: X _(i)=(x _(i,0) ,x _(i,1) , . . . ,x _(i,n)), wherein x _(i,1) , . . . , x _(i,i−m+1) and x _(i,i+1) , . . . , x _(i,n) are all 0; the group controller judges whether X₀,X₁ . . . , X_(n) are linearly independent, if they are linearly independent, then proceed to the step 3; otherwise, return to the step 2, or return to the step 1 to allow the group member to reselect a private vector; the step 3 is implemented by the following: the group controller selects a random number k in the finite field F as a group key, and constructs a system of linear equations by using the new set of vectors and the group key: suppose a₀,a₁, . . . , a_(n) are unknown parameters, and the group controller solves the central vector A=(a₀,a₁, . . . , a_(n)) from the following system of linear equations: $\left\{ {\begin{matrix} {{{x_{0,0}a_{0}} + {x_{0,1}a_{1}} + \ldots + {x_{0,n}a_{n}}} = k} \\ {{{x_{1,0}a_{0}} + {x_{1,1}a_{1}} + \ldots + {x_{1,n}a_{n}}} = k} \\ \ldots \\ {{{x_{n,0}a_{0}} + {x_{n,1}a_{1}} + \ldots + {x_{n,n}a_{n}}} = k} \end{matrix}\quad} \right.$ this system of linear equations is represented in vector form: X×A^(T)=K^(T) wherein the vector K=(k,k, . . . , k), the vector A=(a₀,a₁, . . . , a_(n)), the matrix ${X = {\begin{bmatrix} X_{0} \\ X_{1} \\ \ldots \\ X_{n} \end{bmatrix} = \begin{bmatrix} x_{0,0} & x_{0,1} & \ldots & x_{0,n} \\ x_{1,0} & x_{1,1} & \ldots & x_{1,n} \\ \ldots & \ldots & \ldots & \ldots \\ x_{n,0} & x_{n,1} & \ldots & x_{n,n} \end{bmatrix}}};$ since X₀,X₁ . . . , X_(n) are linearly independent vectors and the determinant of the coefficient matrix |X|≠0, this system of linear equations has the unique solution. the central vector A=(a₀,a₁, . . . , a_(n)) and the mapping parameter r are broadcasted or multicasted by the group controller to all the group members via open channel; the step 4 is implemented by the following: after the group members receive the central vector A=(a₀,a₁, . . . , a_(n)) and the mapping parameter r, the private vector of the group member is mapped to a new vector in the vector space according to the mapping parameter r: for the group member u_(i), where i=1,2, . . . , m−1: x_(i, 0) = f(v_(i, 0), r) x_(i, 1) = f(v_(i, 1), r) … x_(i, m − 1) = f(v_(i, m − 1), r) that is to say: X _(i)=(x _(i,0) ,x _(i,1) , . . . ,x _(i,n), wherein x _(i,m) , . . . , x _(i,n) are all 0; for the group member u_(i), where i=m,m+1, . . . , n: x_(i, 0) = f(v_(i, 0), r) x_(i, i − m + 2) = f(v_(i, 1), r) … x_(i, i) = f(v_(i, m − 1), r) that is to say: X _(i)=(x _(i,0) ,x _(i,1) , . . . ,x _(i,n)), wherein x_(i,1), . . . ,x_(i,i−m+1) and x_(i,i+1), . . . ,x_(i,n) are all 0; then the group members u_(i) calculate the group key k via the equation below, i.e. the inner product of the vector X_(i) and the central vector A=(a₀,a₁, . . . , a_(n)): k=X _(i)×A^(T) =x _(i,0) a ₁ + . . . +x _(i,n) a _(n) wherein T is the transpose of the matrix.
 9. The group key management approach based on linear geometry of claim 1 or 5, wherein the m-dimensional private vector is a two-dimensional private vector.
 10. The group key management approach based on linear geometry of claim 1 or 5, wherein the group key management approach based on linear geometry further includes auto update: if no group member joins or leaves the group in a preset period, then the group controller will update the group key periodically, the group controller reselects the mapping parameter and the group key and computes the central vector, and the central vector and the mapping parameter are broadcasted or multicasted to all group members by the group controller via open channel. 